It is now well known that long lengths of glass fibers have considerable potential strength but the strength is realized only if the pristine fiber is protected with a coating the instant it is drawn. Considerable effort is being expended to develop effective techniques for coating glass fiber waveguides. Most of this activity involves coating the fiber with prepolymer material and curing the prepolymer using heat or light. Coating the fiber with a hot thermoplastic resin is also being investigated extensively. In each case the coating material is typically applied by immersion of the pristine fiber in a reservoir containing fluid prepolymer or polymer material. Typically, the fiber enters the coating fluid through a free surface, and exits through a small die at the bottom of the reservoir. The coating material is cured or cooled to a solid state, and the fiber is taken up by a suitable capstan and reel.
This general technique has been used widely and successfully for coating considerable lengths of high quality glass fiber waveguide. However, significant economical advantage is still to be realized by increasing the draw rate of commercially produced fiber. We have found new difficulties in the coating process as the rate exceeds the conventional rate of approximately one meter per second. For example, we have found that a fiber traveling at a high speed as it enters the free surface of the coating fluid draws a considerable quantity of air into the fluid medium. As coating progresses, the air accumulates in the reservoir in the form of air bubbles. As the quantity of these bubbles builds, some tend to pass through the die and remain in the fiber coating. The number of bubbles or voids found in a given length of coated fiber is believed to be related directly or indirectly to the concentration of bubbles in the reservoir. Moreover, we have found that these bubbles move rapidly with the streamlines in the fluid and interact mechanically with the fiber causing instabilities in the fiber alignment with respect to the die. It is known to be important to avoid excursions of the fiber as it passes through the die. In addition instabilities associated with air bubble entrainment can produce fiber misalignment within the coating as well as coating diameter variations.
In summary we have identified a new and important obstacle to high-speed manufacture of glass fiber waveguide.